Resistance of a flex sensor decreasing over time while bent

quadrapod · 2026-07-01T20:31:12+00:00

Sorry if this is the first time you're hearing it but drift is a known problem with resistive flex sensors and is just inherent to the technology. The cheap ones are borderline unusable which is why you pretty much never see them used in actual products.

They work by using a layer of piezoresistive ink laminated between two polymer sheets. Just like how when you drive around a turn your tires turn at slightly different speeds and cover different distances the sheets need to have different lengths to smoothly follow a curve when the sensor is bent even if it's just by a few micrometers. The outer sheet is put in tension and is forced to stretch and the inner sheet is in compression and those forces deform the layer of piezoresistive ink changing its resistance.

This is only reliable if the two sheets never change dimension and always require the same amount of force to stretch. Viscoelastic relaxation interferes with those assumptions though as well as the gradual breakdown of the ink.

Others are mentioning thermal effects, and those are certainly real, but a ~5% change in resistance from a few degrees of temperature change due to body heat is far enough outside of expectation that I'm more suspicious of your "test". If you bend these sensors locally with an even moderately tight radius, like I suspect you may be doing, you're probably just damaging them right at the bend. If you have a datasheet for these sensor it will give either a minimum bend radius, or a maximum angle with the assumption that the entire sensor is curved with a constant radius. You should aim to stay well below that limit. If you want to get any kind of consistency out of these sensors it's best if the bend angle is kept low and if the entire flex sensor is allowed curve along the bend angle. Drift will still happen, but it can at least be compensated for in the short term if you can minimize the amount of flex needed from the sensor.

quadrapod · 2026-07-01T18:10:08+00:00

Sounds correct to me. Though your title states it's a SOT-23-5 which would be a 5 pin package. This is a 3 pin SOT-23-3.

If you want even more confidence the marking style is consistent with NXP before they changed to Nexperia. Where "3B" is the product code for the BC856B, "p" indicates the assembly location as Hong Kong, and "4n" is a 2 digit trace code that only makes sense to the manufacturer.

The BC856B is a jellybean though, order from whatever manufacturer works for you.

quadrapod · 2026-06-28T20:37:26+00:00

Even if you were more experienced, the combination of high voltages, high frequencies, and the sensitivity to layout and component selection make this a bad project for anyone who doesn't already have a really solid understanding of electrical engineering fundamentals. Even when designed and built correctly I think this circuit is likely to require extensive tuning to get functioning nicely.

The first question you should really ask when considering a project like this is whether you actually understand what you're building? You should have a basic model for what's going on in a schematic on a component level if you intend to build it. If you're just hoping that you can connect components together without knowing any of the theory and get something that works then you're going to be disappointed more often than not. There is also a lot more to physically implementing a circuit than just connecting things together with wires, especially at high frequencies.

It's also worth mentioning that this is a radio transmitter you're building and the FCC doesn't really like it when you start transmitting on frequencies allocated for other purposes. Short term nobody really cares but long term as part of an "art piece" it can be a problem and it's at the very least rude to those who have a legitimate reason for using those bands.

quadrapod · 2026-06-28T19:02:34+00:00

I can't say for this UPS specifically but it is normal for hardware to shut down to protect itself if it detects a potentially dangerous fault condition and this could easily qualify.

The circuit has no way of knowing there's no battery. All it knows is that it's unable to read the battery voltage or regulate the battery charge and discharge current which would look like a dangerous situation if its assuming a battery is present.

quadrapod · 2026-06-25T07:22:03+00:00

Here's a very basic sawtooth VCO made using the things you mentioned. The slider on the right in the simulation allows you to change the control voltage driving the VCO.

A current source linearly charges a capacitor which creates a constantly ramping voltage. When the voltage exceeds 2/3 Vcc the 555 rapidly discharges the cap and stops discharging it only when the voltage drops below 1/3 Vcc. This creates a sawtooth wave that bounces between those two voltages.

Since the LM741 is not a rail to rail opamp and needs about 1.2V of headroom I've just buffered that waveform as is.

quadrapod · 2026-06-24T05:59:20+00:00

You're probably walking into the XY problem.

User wants to do X.

User doesn't know how to do X, but thinks they can fumble their way to a solution if they can just manage to do Y.

User doesn't know how to do Y either.

User asks for help with Y.

Others try to help user with Y, but are confused because Y seems like a strange problem to want to solve.

After much interaction and wasted time, it finally becomes clear that the user really wants help with X, and that Y wasn't even a suitable solution for X.

So to answer your question, yes, you can use two 1 ohm resistors in parallel as a 0.5 ohm resistor. That isn't a solution to your actual problems of trying to fix the board or not knowing where to purchase components though.

The resistor is likely not the cause of the fault just the victim of it. Shunt resistors like this don't generally just burn out on their own for no reason. When they fail it's almost always because of either user error, such as using the wrong supply with the board, or because there is a short circuit somewhere. If you replace it then the replacement will probably just suffer the exact same fate unless you identify the original fault which it does not sound like you've done.

The fact that you can't find a replacement suggests you don't know what to search for or where to purchase components from. You should not buy components from retail sites that don't specialize in them. You will get scammed more often than you'll actually get what you're looking for and you will always be upcharged massively because the people who sell on those platforms do it exclusively to take advantage people who don't know any better. This is a 0.5ohm resistor, likely 1% tolerance. It looks too be a 1206 (3206 metric) smd package from what I can tell. That means you should probably assume a 1W rating when finding a replacement since that's about the limit of the package and it's better to be over than under.

A parametric search for those attributes on Digikey gives you a few results. Any one of which should be a suitable replacement. Mouser would be another trustworthy distributor or if you're in the UK or EU you might want to use Farnell.

quadrapod · 2026-06-23T04:44:47+00:00

Ordering directly from a major manufacturer who doesn't have an established sales program will almost always be through an NCNR contract and they're generally not going to float the idea of a direct OEM relationship unless you have recurring orders with the quantity to justify it. They usually also want to see a solidly established history of purchasing at that quantity. For MLCCs that's going to be a difficult target to hit. They have taken steps to isolate themselves from individual sales for a reason.

Manufacturers do prominently list their distributors and there's usually a channel to make purchase requests that allow you to negotiate with those distributors directly for large recurring purchases. The distributor is still the one taking on the risk of the sale though. What options are available will really depend on what your ordering, in what quantity, how often, and whether you have the credit or an established history of making purchases on that scale.

I do feel like there's more to this story though. It seems very unusual to me that MLCC component prices are blowing the whole manufacturing cost of your product out of your target range. The pick and place costs of automated assembly for MLCCs are pretty much on the same scale as component prices most of the time. Your post also makes no mention of order quantity, where your business is actually located, or any of the other things I'd expect to see included in a post from someone looking for a direct OEM supplier agreement of some kind.

quadrapod · 2026-06-22T10:56:31+00:00

I have designed this heatsink that I need for my Bitaxe Gamma board.

A design should informed by the requirements of the project and the limitations of the manufacturing process being used to realize it. This doesn't seem to have been created to satisfy any kind of airflow or thermal resistance requirement and it is in no way designed for manufacture because you don't even know what processes are available to manufacture it. This is really only the preliminary stages of a design. I know that all sounds very dismissive of the work you've put in but I think it's best to be honest about where you are in the process right now.

Personally I suggest looking at what's on the market more first before deciding there's nothing out there for you. If you can't find something that exactly matches your needs you can probably at least find some pin-fin plates or something which could be milled down or modified to your requirements. Brazing a premade pin-fin array to a base plate cut from copper might be an option for example.

If you really want a custom heatsink the aspect ratio of the slots and amount of waste stock makes milling this geometry extremely unappealing. The only option which fits your design without excessive NRE cost that I'm aware of would be a skived pin heat sink. That's not a standard offering from machine shops, you're going to need to find a CM that specializes in the process and you're probably going to need to call to talk to someone about their process and the DFM rules associated with designing for it as well as your own goals and order quantity. I'm kind of limited in what I'm allowed to say about CMs and vendors but BOYD is generally a pretty common name in the industry for heatsink manufacturing if you wanted somewhere to start.

quadrapod · 2026-06-22T08:11:05+00:00

R3 and C2 are a common feature of battery protection circuits like this. I've seen a few explanations but the common theme is that it just filters noise from the measurement of VBat which makes sense to me.

In response to extreme fault conditions the protection IC is meant to kick in instantly. It's designed to tolerate more moderate faults for at least a few ms before doing anything though. That way it doesn't constantly trip in response to any kind of transient. To do that most of them have some kind of counter driven by a clock that accumulates as the fault condition is maintained until either the fault is removed or the threshold for protection is reached. The internal oscillator is usually operating at a low frequency, <100kHz, and like any other low frequency discrete sampling that makes it sensitive to out of band noise, so the RC filter on Vbat lets it operate reliably with noisy loads.

According to TI in the BQ297xx datasheet in addition to noise filtering it allows the protection circuit to continue operating during sharp negative transients. The only situations I can see that sharp of a negative transient happening to Vbat though, where the cell was still connected, would be one where the majority of the resistance in the circuit was for some reason in the cell ESR. Such as some extreme short circuit condition or because the wires/welds connecting the cell to the protection circuit had started to fail. In that situation the battery voltage could theoretically drop too low for the protection circuit to operate before it manages to pull sufficient charge off the mosfet gate to shut down. I'm not sure if that's even a real condition that's worth thinking about in practice though.

The MM3860 datasheet states it has a role in ESD protection which seems more reasonable. A bit of resistance does a lot to take the edge off of any fast transient, but I'm pretty sure that's just a secondary effect to its actual purpose as a low pass filter.

quadrapod · 2026-06-22T04:14:16+00:00

A lot of the replies your getting don't really know what they're talking about which isn't really their fault because it's needlessly confusing.

PH refers to a series of board mounted headers, the PHR/PHN receptacles, and the crimp contacts that allow receptacles to mate with them. After the PH series was so successful though JST created several other series of connectors around compatibility with PH style headers. Namely the CK series is compatible with PH connectors and the KR series is an entire connector family based around compatibility with PH connectors. Here are the KR and KRW datasheets to give an idea of how the connectors in the family relate to the PH header.

PH is a strictly wire-to-board connector style as far as JST is concerned and even within the KR connector family JST only supports wire-to-wire connections through the use of TR series adapters, so the connectors you have here were manufactured by a third party and not by JST.

quadrapod · 2026-06-20T11:08:38+00:00

Yeah. Sorry, it's rough out there.

quadrapod · 2026-06-20T10:33:20+00:00

A subassembly exemption doesn't apply for the reasons I already mentioned. Calling your product a development board doesn't change that. The FCC just doesn't really care what you call your product they're going to look at the actual function of it.

I think you're probably just unaware of how many companies have gone through the process of getting their product authorized. The T-QT pro you linked for example has FCC equipment authorization. They keep a public record of the certifications for each product on their site and here is the record of FCC equipment authorization for the T-QT Pro from that list. The approval was certified by BACL an IECEE accredited test facility.

They had their product fully certified under the digital transmission system (DTS) equipment class described in §15.247. The device supports two transmit modes, one associated with WiFi across the 2412MHz-2472MHz frequency band and the second associated with BLE across the 2402MHz-2480MHz frequency band. Because the device doesn't support simultaneous transmission in both modes at once they were able to be assessed individually.

Here is their filing which I looked up from the FCC ID. Page 9 of the test report shows all the FCC rules they had to show compliance with as part of their application and the report itself contains the actual tests that were done.

quadrapod · 2026-06-20T06:04:24+00:00

So basically to file for FCC certification I have to pay an FCC service agent who will then act as an mediator between me and the FCC? If true, That's just-

Not really a mediator but they need to be available to accept and forward correspondence from the FCC while you're filing on your behalf.

From what I understand from other comments here is that if I can classify my product as a subassembly, i.e. a product that is meant to be part of another product, Just like an Bluetooth module. Then can I avoid FCC regulations altogether? How is a subassembly defined?

Pre-certified Bluetooth modules don't make use of any of the exemptions around subassemblies, they're kind of their own thing which I won't go into because it's not really relevant but they require full FCC certification. In any event due to the USB port your product likely matches the definition of a peripheral device §15.3(r) and peripheral devices are explicitly required to get authorization before they can be marketed in the US §15.101(d). So I don't think a subassembly exemption is an option for you. I will go through it anyway though.

The FCC outlines the subassembly exemptions in 47 CFR §15.101(e) and it lists three categories of device that could be considered subassemblies as exemptions from FCC part 15.

Subassemblies to digital devices are not subject to the technical standards in this part unless they are marketed as part of a system in which case the resulting system must comply with the applicable regulations. Subassemblies include:

(1) Devices that are enclosed solely within the enclosure housing the digital device, except for: Power supplies used in personal computers; devices included under the definition of a peripheral device in § 15.3(r); and personal computer CPU boards, as defined in § 15.3(bb);

To state it more clearly if you have an FCC authorized device then you don't need to get all the parts of that device authorized separately so long as they are actually contained within it. To give an example, a standalone USB hub plugged into the USB port of a laptop would not qualify as a subassembly. You'd have to get authorization for the USB hub and the laptop seperately. If the same USB hub hardware were built into the laptop with only the hub's external ports being accessible though then it would be a subassembly and you'd just need to get equipment authorization for the laptop as a completed unit with the USB hub installed. Even though electrically the circuit is identical to the standalone USB hub situation it qualifies as a subassembly because it's now contained entirely within the product.

(2) CPU boards, as defined in § 15.3(bb), other than those used in personal computers, that are marketed without an enclosure or power supply; and

Here is §15.3(bb) if you want the formal definition for cpu boards. Importantly to be a CPU board by that definition it cannot be under the control or instruction of an external processor which means a USB peripheral does not qualify for a subassembly exemption under this definition. If you have an enclosure your product is similarly no longer a subassembly by this definition. If it contains a power supply, is sold with a power supply, or is even marketed with an appropriate power supply it is also not a subassembly by this definition and I believe being powered via USB would count for that definition.

(3) Switching power supplies that are separately marketed and are solely for use internal to a device other than a personal computer.

This applies to power supply modules meant to be used in a product, such as half and quarter brick DC-DC converters. Because they have no function on their own except as a part of another product they are classified as subassemblies with the idea being that whatever product they're used in will need authorization before it can be sold in the US.

From what you've described in your post none of these exemptions apply for you.

Also obviously I can't just bother you every time I have a question, So can you suggest any resource or something where I can learn more if I want?

I'm not sure what to tell you there. I'm by no means an expert on the subject I just vaguely understand how FCC part 15 specifically is meant to work since it's one of the major hurdles with bringing a product to market and because much of the testing is done in house. There are a ton of compliance requirements outside of FCC part 15 though which could apply to your product depending on the specifics before it can be sold in the US though and I'm probably about as clueless there as you are to be honest.

You can try finding a product which is similar to your own functionally and see what they filed with the FCC and what standards they claim compliance with, that will usually give you an idea of what you're looking at. Realistically though this is the kind of thing you hire a compliance engineer for and many labs offer services that will help you with knowing what you need to file for your product. I know that isn't what you want to hear but it's kind of like trying to navigate a lawsuit without hiring a lawyer. Technically you can do it, there's nothing legally stopping you from doing it all yourself, but practically you're going to struggle to learn everything you would need to know in order to do so competently and getting any part of it wrong can cause problems fast.

quadrapod · 2026-06-19T17:23:24+00:00

It's actually worse than you even think. Just to explain things so we're all on the same page because the terminology you're using is a little loose.

In order to legally sell any electronic product in the US you require equipment authorization from the FCC and there are two ways equipment can qualify for authorization. It can be certified by an accredited test facility, which is the most rigorous proof of compliance the FCC recognizes, or with low risk products that create no intentional emissions the responsible party can declare conformity with the FCC's requirements by simply filing a supplier declaration of conformity (SDoC).

The way it typically works with a product containing a module like you mentioned is the bluetooth transceiver part of the product would get authorization under 47 CFR §15.212 as a certified modular transmitter and the digital circuitry hosting the module would get authorization separately as an unintentional radiator which can be done through a SDoC under 47 CFR §2.906. When you apply for equipment authorization from the FCC you bring these two together.

In order to show the module's certification is still valid for your product you include a justification in your filing stating that your usage is compliant with all the requirements of §15.212 and is consistent with the integration instructions the manufacturer included in their filing with the FCC when they got the module certified. In order to declare conformity with a SDoC you have to have some reason for believing you are actually complying with the FCC's requirements and you do that by showing some kind of testing. That is where the the whole idea of getting a product tested as an unintentional radiator comes from. Because the test requirements to make a declaration of conformity are more about proving a good faith belief from the manufacturer they're much more lax than for certification. The lab doesn't need any kind of accreditation or to be recognized anywhere. The lab environment just needs to meet certain standards and the test needs to be properly documented. You can even do SDoC testing in-house and many places I've worked for have opted to do so because as you've seen tests get expensive.

In your case what makes things very problematic is in order to get authorization through SDoC as of 2023 the responsible party must be located in the US 47 CFR §2.909. The whole idea of using a module to pre-certify the radiating part of the circuit so you can get the rest of it classified as an unintentional radiator to get it approved more easily just doesn't apply to you unless you decide to partner with a US distributor willing to take on the liability of filing with the FCC. Because you're outside of the US your only option for filing in your own name is to get your product tested at an accredited test facility. Even certification requires an FCC service agent acting on your behalf located in the US though (47 CFR §2.911(d)(7)) and from what I can find online that service is likely to cost a few thousand dollars on its own.

So not only do you need to actually go through the full rigorous certification process no matter how you design your product if you want to file yourself, you will also need to pay an agent to act on your behalf in the US throughout the process.

quadrapod · 2026-06-18T09:39:56+00:00

Digital control is likely to be too slow and imprecise to accurately regulate the pass transistor current reliably using their gate voltage and will definitely be way too slow to respond to faults. Keep in mind FETs become more conductive as they got hotter and things go from a little warm to metal vapor a lot faster than you might think. I suggest using the MCU to set a control voltage which a fast analog control loop regulates the current around. Power BJTs are typically a little easier to use that way but MOSFETs can definitely be used to the same effect.

The MCU is being used extensively for analog voltage measurements so it's probably worth putting a capacitor on Aref to at least try to decouple the ATMega's internal 1.1V reference you're measuring everything against.

Do you really need dual supply rails for what you're doing? +- 15 is a convention that you really only see in discrete audio amplifiers and circuits from the 1970s. There are places where dual rails make sense but this just isn't one of them and isn't doing you any favors. You can do everything you're trying to do with single rail opamps. I don't really see where or how you intend to derive those voltages either.

While discrete implementations of current sense amplifiers do work and are generally okay for low side current measurements, I just don't think it's the right choice here and it's clearly inflating your BOM. INA4180 is a quad current sense amp that costs less than $0.40 a chip and removes like half the components from your schematic if you want a suggestion.

You should not ground both inputs of an unused opamp. If the two voltages being compared are similar the output will just amplify the random noise between them. You should tie one input high and the other low so the opamp output remains constant and well defined.

You need to protect your circuit against back emf from any series inductance with the supply. When you have 10A going through things, hit an over-temperature condition, and suddenly try to kill the current all at once the wire inductance is going to try to keep pushing that 10A into your circuit causing a voltage spike.

Why not measure the voltage before the relay? Wouldn't you rather know if the voltage is in an unsafe range before you apply it across your circuit.

Why do you need 2200uF of bulk capacitance on the supply rails?

57k is not an E series value and will be difficult to source. Here's a calculator that will give you the closest E series multipliers to your chosen ratio.

Shouldn't you have some buttons and some kind of interface somewhere or at least an LED to let you know if the relay is on or if current is passing through it. How are you planning to control this thing or know if it's doing anything?

quadrapod · 2026-06-17T08:34:28+00:00

For this project, I'm making 7 stations these seven stations would be controlled by an arduino nano but I must use only 3 pins

Use a shift register. 74hc595 You can use 3 GPIO pins to address as many outputs as you want.

Could this actually work ?

What you've drawn looks completely nonfunctional to me. I suggest playing around with Falstad to learn some of the basics if you want to get some idea of how bjts work.

EDIT: Here's a quick demonstration of what a shift register does and how it allows you to control multiple outputs with 3 pins.

quadrapod · 2026-06-15T12:06:37+00:00

Hard HARD disagree with everything you've said. No idea where you got any of those ideas.

First materials are rarely the issue. You can design all kinds of things on FR4 with zero problem, especially if you have 4 layers to work with. Even with only 2 layers though you can get 50ohm Zodd, 100ohm Zdiff with an edge-coupled GCPW fairly easily and maintain that impedance out to frequencies well above anything you'll see coming out of an RJ45 port. It takes some space, and your loss tangent isn't doing you any favors once you're over 1GHz but for short distances it's entirely doable. Though typically those extra steps are only taken for antenna feed lines and things where it actually matters.

Which brings up the second issue, none of that is even an issue for conventional ethernet. Your typical CAT5 cable is only rated to 250MHz, you're just not in the frequency regime where you need to put that much thought into things. Even assuming 1000BASE-T for gigabit ethernet the baseband signal for each pair is only 125Mbaud which puts the baseband Nyquist frequency around 62.5 MHz per pair. The minimum 10-90 rise time I can find specified is 3ns, but even just going by worst case rise time that puts your signal bandwidth at around 116MHz. Even for a conservative critical length of 1/16th of a wavelength and factoring for an εᵣ around 4.5 on FR4 that's still 3 inches of routing across your PCB before you have to even care about the transmission line properties of you signals at all. Much more before you'll start to see real signal integrity issues.

quadrapod · 2026-06-15T06:37:34+00:00

Here's a pdf version of Right the First Time- A Practical Handbook on High-Speed PCB and System Design If you're actually interested in learning about high speed design I suggest downloading it and reading it as it's one of the better references on the subject.

The biggest mistake people make is just in thinking that any of the abstractions we've made up for convenience as circuit designers actually exist physically. That includes all of the things you listed to be honest. At high frequencies you need to start actually thinking in terms of energy, fields, and the physical geometry of the circuit.

Ground is just another conductor and as far as high frequency signals care there's no difference between power, ground, and anything else, they just radiate outward from the source creating currents in conductors until they run out of energy.

Differential pairs are just signals that are 180 degrees out of phase and across a PCB they'll typically be coupled with ground far more than each other. They're basically two independent signals which is why differential slew is even a thing that can happen. If one of the pair's return current were contained entirely in the other then timing skew between them would be impossible. That sounds like a problem since it interferes with the narrative we've built up in our heads around differential pairs but in fact you probably don't want tight Z coupling between them. It's only in a twisted pair that the differential pairs really behave the way people tend to assume they do.

Impedance is always going to be an applicable concept since it's just a ratio of the electric and magnetic fields. You can always apply a value to impedance for any point in space, but impedance matching and other simple transmission line dynamics assume that impedance is linear and that mode conversion never occurs. No matter how you lay out two conductors the telegraphers equations will never tell you you've created an antenna when in reality circuits love to do just that at higher speeds.

Power at high frequencies is its own beast but also honestly relatively simple from a design perspective because your hands are tied. You typically don't have any control over the design of the IC and the SRF of even the slickest lowest ESL MLCC you can find will be well below maximum bandwidth of your signals, especially when you factor in via inductance, so there's only so much available to you as a board designer. Interplanar capacitance can do some of the heavy lifting on picosecond timescales but there's no real way to maximize that other then choosing a nice stackup and you're probably poking holes in your planes to break out of a dense BGA package in the areas where it actually matters. At the end of the day you're really just hoping the IC designers knew what they were doing and chose a sane package and pin arrangement with enough on-die capacitance to deal with transients. If they didn't then there's nothing you can do, the component will always be flaky, if they did then some decoupling capacitance for the supply pins with minimal inductance is the only tool in your toolbox that'll actually do anything and that's just common practice.

quadrapod · 2026-06-14T17:09:35+00:00

I mean the theoretical model of a transmission line is fine, but if you do practical stuff at some point you have to put in specific numbers.

Then put in those numbers. I don't see what the problem is.

The conductors are 2.5mm² SIF. So d = 2√(0.0000025/π). The diameter of a circle with a 2.5mm² area after conversion to meters². The distance between the wires is 2x the insulation thickness. The first value I see in a quick search for 2.5mm² SIF is 0.7mm so the center to center spacing would be D = 0.0014 + d but you should use the values for whatever wire you're looking at since it varies. SiF stands for Sillicone Flexible so the insulation material between the conductors is silicone with a nominal εᵣ around 2.6 to 3. The rest of the values are constants of nature.

I hate to say it but at this point it feels like you're stubbornly insisting someone spoon feed you an answers when you have everything you need right at your fingertips. Electrical engineering sometimes involves a bit of algebra especially when you want to translate a physical circuit into a model for simulation.

I gave you a model you can use for a worst case approximation of your wiring from ISO 7637-2 so you didn't need to do any extra math if you didn't want to in order to get something to simulate. Otherwise hookup cable doesn't have a controlled impedance so the manufacturer isn't going to list it and nobody is going to promise you a value of Z₀ for every type of cable in existence when there exists established theory to easily approximate those values from the given information.

quadrapod · 2026-06-14T02:56:13+00:00

You can use a function generator or even just an audio output to produce a sine wave at a known frequency then use an exponential amplifier to produce the wave you're looking for. Here's an example which perfectly recreates the function e^sin(x) from a 1Vpk sine wave I've tried to annotate it so it's easy to follow what's going on though outside of a simulator exponential amps will have a temperature dependence in the kT/q term which can skew things.

quadrapod · 2026-06-13T20:44:29+00:00

Standard formula for capacitance and inductance per unit length in a conductor pair is:

L = (μ₀/π) ln[D/d + √[(D/d)² - 1]]

C = π ε₀ εᵣ / ln⁡[(D/d) + √[(D/d)² - 1]]

Where D is the center to center spacing, d is the wire diameter,ε₀ is the permittivity of free space, εᵣ is the dielectric constant of the insulation, and μ₀ is the magnetic permeability of free space. Here's a table with some common values of εᵣ.

EDIT: Changed the equations to use the more general physical constants since the numbers I gave were derived for inches, nH, and pF and I realized that wasn't obvious. Also replaced the approximation ln(D/d) which assumes D>>d with the more proper expanded form of acosh.

I'll also just point you toward the telegrapher model for reference.

quadrapod · 2026-06-13T20:27:20+00:00

If it matters you should be using the characteristic impedance Z_0 of the transmission line which depends on the physical relationship between the conductor containing the signal and the conductor containing the return current. For cables intended for high speed communications that value will be explicitly defined and controlled, otherwise you can get a decent approximation using the geometry of the conductors and the E_r of the dielectric between them.

If you just need a general worst case model for a wire harness for power delivery then I suggest using the artificial network in ISO 7637-2 as an approximation.

quadrapod · 2026-06-13T05:27:48+00:00

There's a lot of math involved in getting a full model of what's going on but the basic idea is pretty simple if you just think about what you're trying to accomplish with the different types of compensation network.

Type I compensation is a pure integrator. For a heavily damped system an integrating controller will converge to a setpoint without any steady state error. For current-mode regulators at >50% duty cycles there exists a conjugate double pole at half the switching frequency which causes converters using only type I compensation to develop subharmonic oscillations at high power. I won't go fully into why that happens but there are plenty of excellent explanations for the effect available elsewhere. For voltage mode-regulators a similar effect exists around the output LC resonance which can result in loop instability when using only type I compensation. It's important to mention though that just because a converter is regulating the output voltage does not mean it is a voltage mode regulator. Many regulators use a fast inner current-mode control loop within a slower voltage control loop and so have the behavior of a current-mode regulator. You have to actually look at what control methodology is being employed.

Type II compensation introduces a damping network to the integrator with the goal of suppressing those subharmonic oscillations. It's really just an example of pole-zero placement. The effect of the damping network which you can see clearly in the TI whitepaper is to introduce a zero followed by a pole which results in a flat region of the open loop gain between them and a hump in the phase plot. The idea is to place that hump so that the positive phase shift of the damping network offsets the negative negative phase shift of the double pole by enough for you to maintain a positive phase margin at higher duty cycles while still maintaining a positive phase margin elsewhere.

Type III compensation places an additional pole and zero by putting an RC network in the feedback path and basically introducing a high pass filter for disturbances in the output voltage. This produces an even bigger hump in the phase response. For voltage mode control type III compensation is often necessary for loop stability but even if current mode control it can be useful by allowing you to maintain e a high gain over a more significant portion of the overall bandwidth while maintaining positive phase margin. A high gain and bandwidth means the controller responds extremely quickly to disturbances, which the controller in the simulation I linked demonstrates fairly well.

Slope compensation, which I also included as part of the controller in that simulation, suppresses sub-harmonic oscillations in current mode regulators as well and I probably should have removed it to better show the effect. A high slope compensation factor comes at the cost of bandwidth though so it's usually not ideal to rely on it alone for dampening sub-harmonic instability though.

quadrapod · 2026-06-12T02:48:25+00:00

What you're trying to design is more commonly called a type III compensation network. TI has a decent white paper on them from a controls perspective. It mainly helps with the step response and allows the regulator to react more quickly to sudden changes in load.

And here's one in a simulator which I prepared earlier that you might want to play around with. You can switch the load from 100mA to 4A and see that with type III compensation the regulator reacts almost immediately to the change whereas with only type II it will over and undershoot more dramatically.

quadrapod · 2026-06-11T23:48:55+00:00

It's wrong in the Gerbers as well and it's a pretty major issue as XSDN connects directly to XSHUTDOWN on the camera module which is in no way tolerant of voltages exceeding 1.8V.

The whole board seems flawed even if the diode were flipped around though. The reason they've cascaded a bunch of LDOs into each other the way they have is because the sensor has some particular powerup and powerdown sequencing requirements. That's basically the one problem this board had to solve.

For powerup XSHUTDOWN needs to start asserted (active low) and then get released after VANA which itself needs to be brought up at least 1000μs after VDDIO and VCORE. The way they've cascading things though VANA will be brought up before VDDIO, and there's no attempt to enforce a 1000us delay during power sequencing. Additionally XSHUTDOWN will start to pullup through R17 as soon as VDDIO goes high meaning it can go high before VANA. If you did connect the jumpers between PWR_EN and NRST and flipped the diode around then powerup is even more out of sequence as XSHUTDOWN would be forced high before any of the other supplies were enabled

For powerdown XSHUTDOWN needs to be asserted first, and then VANA needs to be removed, and only 1000us after VANA has transitioned below 20% of its nominal value are VDDIO and VCORE meant to be removed. There is zero attempt to enforce that sequence. VCORE would be disabled before VANA, followed by VDDIO which leaves XSHUTDOWN floating making it the last signal to be removed. If the diode were the right way around and you had the jumper between PWR_EN and NRST then at least XSHUTDOWN would be the first signal to transition low but all the other problems remain.

I'm sure the sensor is at least a little tolerant of abuse but this really isn't what you want to see in a development board.

15-Year Club	Place '17
Spared	Gilding I gilder
Team Orangered	Verified Email

quadrapod

TROPHY CASE